Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. After passing through a leak, a test sample containing helium is drawn into a leak detection instrument and is measured. An important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input test sample is ionized and mass analyzed by the spectrometer tube in order to separate the helium component. In one approach, a test piece is pressurized with helium. A sniffer probe connected to the test port of the leak detector is moved around the exterior of the test piece. Helium passes through leaks in the test piece, is drawn into the probe and is measured by the leak detector. In another approach, the interior of the test piece is coupled to the test port of the leak detector and is evacuated. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
One of the difficulties associated with helium mass spectrometer leak detection is that the inlet of the mass spectrometer tube must be maintained at a relatively low pressure, typically 2×10−4 Torr. In a so-called conventional leak detector, the test port, which is connected to the test piece or to the sniffer probe, must be maintained at relatively low pressure. Thus, the vacuum pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, it may be difficult or impossible to reach the required pressure level. If the required pressure level can be reached, the pumping cycle is lengthy.
Techniques have been proposed in the prior art to overcome this difficulty. A counterflow leak detector disclosed in U.S. Pat. No. 3,690,151, issued Sep. 12, 1972 to Briggs, utilizes a technique of reverse flow of helium through a diffusion pump to the mass spectrometer. The leak detector test port can be operated at the pressure of the diffusion pump foreline. A similar approach utilizes reverse flow of helium through a turbomolecular pump. A technique for gross leak detection is disclosed in U.S. Pat. No. 4,735,084 issued Apr. 5, 1988 to Fruzzetti. The tracer gas is passed in reverse direction through one or two stages of a mechanical vacuum pump. These techniques have permitted the test port pressure to be higher than for conventional leak detectors. Nonetheless, reaching the higher test port pressure can be difficult when testing large volumes, dirty parts or parts with large leaks.
A simplified schematic diagram of a prior art leak detector for large leak testing is shown in FIG. 1. A test piece 10 is attached to an inlet flange 12. Inlet flange 12 defines a test port of the leak detector and is connected through a test valve 32 and a differential pressure aperture 30 to a test line 14. The leak detector includes a forepump 16, a roughing pump 18, a turbomolecular pump (turbopump) 20 and a mass spectrometer 22. A foreline 24 of turbopump 20 is connected to test line 14, and mass spectrometer 22 is connected to the inlet of turbopump 20. A midstage line 26 of turbopump 20 may be connected to test line 14. Forepump 16 rough pumps test line 14 and test piece 10 from ambient pressure and also backs turbopump 20. Helium that enters the test port from test piece 10 flows in contraflow or reverse direction through turbopump 20 and into mass spectrometer 22. The mass spectrometer detects the helium and indicates a helium leak rate. An alternate prior art non-contraflow configuration uses a direct connection 34 between test line 14 and the inlet of mass spectrometer 22.
For large leak testing, where the test port pressure may be greater than the allowable foreline pressure of the turbopump 20, roughing pump 18 is utilized in prior art leak detectors with a roughing line 28 and a roughing valve 36. Aperture 30 operates such that most of the gas flows to roughing pump 18 while a fraction of the gas flows to the forepump 16, with helium passing in reverse direction through mass spectrometer 22. A bypass valve 38 is used to bypass aperture 30. Testing with two pumps and a differential pressure aperture is inherently unreliable since, for example, the aperture can become partially plugged by contamination, resulting in erroneous readings. Furthermore, the cost of the roughing pump and associated hardware significantly increases cost.
European Patent Application No. 0 352 371 published Jan. 31, 1990 discloses a helium leak detector including an ion pump connected to a probe in the form of a silica glass capillary tube. The silica glass tube is heated to a temperature between 300° C. and 900° C. and thereby becomes permeable to helium. U.S. Pat. No. 5,325,708 issued Jul. 5, 1994 to De Simon discloses a helium detecting unit using a quartz capillary membrane, a filament for heating the membrane and an ion pump. U.S. Pat. No. 5,661,229 issued Aug. 26, 1997 to Bohm et al. discloses a leak detector with a polymer or heated quartz window for selectively passing helium to a gas-consuming vacuum gauge.
All of the prior art helium leak detectors have had one or more drawbacks, including limited pressure ranges, susceptibility to contaminants and/or high cost. Accordingly, there is a need for improved methods and apparatus for leak detection.